2-methylalkanals are useful as intermediates for various commercial processes; e.g. they may be oxidized to the corresponding carboxylic acids which, in turn, are used as reactants in the manufacture of perfumes. The compounds obtained by the method of the present invention are also useful for the production of pharmaceuticals and plant protection agents.
The principal process for the production of aldehydes having more than two carbon atoms is the well known hydroformylation reaction. This comprises the addition of carbon monoxide and hydrogen to olefins in the presence of metal carbonyl compounds as catalysts. Thus far, only cobalt and rhodium have been found suitable for this purpose.
This reaction generally produces a mixture of isomeric aldehydes. In particular, n-aldehydes and their isomers, as well as .alpha.-methylaldehydes are formed from olefins having unsaturation on their terminal carbon atoms. In actuality, the only way to avoid the formation of mixtures is to use symmetrical or sterically inhibited olefins as starting materials.
The ratio of n-compounds to corresponding iso-compounds in the formylation reaction product varies within wide ranges. This is determined by the particular catalysts and the reaction conditions used.
A further complicating factor resides in the source of the olefinic starting materials. They normally come from the processing of crude oil and frequently contain mixtures of olefins, rather than individual compounds in a pure or nearly pure state; such mixtures usually contain olefins having the same number of carbon atoms, but with different structures.
For example, when mixtures of 2-methylbutene-1 and 2-methylbutene-2 are hydroformylated, the resultant products include 2,3-dimethylbutanal, 3-methylpentanal and 4-methylpentanal. Correspondingly, mixtures of 3-methylpentene-2 and 2-ethylbutene-1 will yield 2,3-dimethylpentanal, 3-ethylpentanal, and 4-methylhexanal.
Heretofore, the aldehyde mixtures referred to were separated by fractional distillation. However, this presents a problem because most of these aldehydes are extremely sensitive to oxidation. They tend to condense to form higher molecular weight secondary products and to decompose thermally.
Therefore, it is necessary to observe a number of precautions in carrying out these distillations. As would be expected, the sensitive aldehydes are distilled at as low a temperature as possible and only after complete removal of the catalysts. Variations of the process use azeotropic and extractive distillation methods in order to avoid the unwanted decomposition and/or condensation.
The separation is made more difficult by the fact that the hydroformylation reaction results in aldehydes having the same number of carbon atoms and, therefore, they possess boiling points which differ only slightly. To successfully separate these compounds by distillation, it is necessary to use columns having extremely high selectivity and to use very high reflux ratios. Such requirements are not compatible with cost-effective separation and there is often a substantial reduction in yield due to the fact that, in spite of all the precautions, higher boiling compounds are formed from the n-aldehydes which are of low thermal stability.